Epoxy compounds are widely preferred as die-attach, bonding and encapsulating adhesives in the microelectronics industry due to their good adhesion to a broad spectrum of materials. Although conventional epoxy compounds are inherently rigid and brittle, making them susceptible to the stress of thermal cycling in manufacturing procedures and potentially leading to adhesive failure or fracture of the die or chip, recent advances in formulating epoxy adhesives have resulted in epoxies that are flexible and resilient. According to the principles outlined in "Advances in Custom-Formulated Flexible Epoxies", by R. D. Hermansen and S. E. Lau, Adhesives Age, 38, July 1993, flexibility can be obtained in a polymer by the absence of ring structures, low levels of crosslinking, and free rotation about polymeric bonds. Carbon-carbon double bonds adjacent to carbon-carbon single bonds, and ester and ether groups enhance free rotation.
Glycidyl esters, which are the reaction products of glycidyl alcohols and mono- or dimer carboxylic acids, are flexible and particularly suitable for use as the epoxy base in microelectronics adhesives. The prior art has looked at several methods to prepare glycidyl esters, but these methods are not without disadvantages when the product will be used in electronics devices.
For example, the synthesis of these compounds can be accomplished by glycidization of carboxylic acids (G. Maerker; J. Org. Chem., 26, 2681 (1961)), or carboxylic acid salts (G. Maerker; J. Am. Oil chem. Soc., 38, 194 (1961)) with epichlorohydrin in the presence of sodium hydroxide, or by glycidization of carboxylic acid chlorides with glycidol (E. B. Kester; J. Org. Chem., 550 (1943)). These methods leave high levels of residual chlorine, chloride and alkali metal ions, which, when used in adhesives for microbonding in electronics devices, tend to promote corrosion of electrical leads in the vicinity of the adhesive.
Glycidyl esters have also been prepared by the transesterification of carboxylic acids using an alkali metal halide (Japanese patent 55127389), such as sodium bromide, or thallium compounds (Helvetica Chimica Acta, 60, 1845 (1977)), but halide ions are potentially corrosive in microelectronics applications, and thallium compounds are known to be extremely toxic (Merck Index).
In still another method, glycidol can be reacted with an acid anhydride; however, the result is that one molecule of acid is formed as a by-product for every molecule of glycidyl ester (M. Lok; Chem. and Phys. of Lipids, 36, 329 (1985)).
U.S. Pat. No. 5,036,154 describes the preparation of glycidyl esters by the oxidation of allyl esters employing hydrogen peroxide with salts of tungstic acid and phosphoric acid, and a phase transfer catalyst. This method requires a long reaction time, and residual phase transfer catalyst can be difficult to remove and can cause premature polymerization.
A dehydration system using N,N'-carbonyldiimidazole (J. C. S. Perkin I, 538 (1977)) is known to effect ester formation from carboxylic acids and alcohols, but the imidazole by-product will react with oxiranes at room temperature.
Carbodiimides have been described as acylating agents for esterification (Chem. Rev., 81, 589 (1981)); they have not, however, been considered to be very useful for this purpose because side products formed from the rearrangement of the 0-acyl isourea intermediate reduce the yield of the desired product. If catalytic quantities of a pyridine derivative are used in the reaction, the yield of ester can be enhanced.
In Japanese patent application 05 59,031, a carbodiimide reagent and pyridine-type catalyst are used to condense glycidol with a carboxylic acid to afford a glycidyl ester. This method is specific to amino acid containing compounds used for their liquid crystal properties, which are known to impart rigidity and stiffness to a system. These compounds can be recovered as precipitated crystals. Flexible epoxy compounds, in contrast, generally take the form of oil, and are not easily recovered from the reaction medium. Furthermore, many of these flexible epoxies are derived from dimer acids and other fatty acids and tend to form severe emulsions when an aqueous extraction is performed to isolate the product. The emulsions also prevent the complete removal of the pyridine-type catalyst, which if not removed, over time will react with the epoxy, increasing the viscosity, decreasing the oxirane content, and sometimes causing the product to form into a gel.
Another requirement for an electronics adhesive is that the thixotropic index be high and remain stable over a prolonged period (usually about 24 hours). The thixotropic index is the ratio between the viscosity of the adhesive at high shear and that at low shear. In many microbonding operations, the adhesive is delivered by syringe under shear. The higher the thixotropic index of the adhesive, the cleaner its delivery from syringe to substrate because the viscosity of the adhesive increases immediately upon dispensing as the shear is reduced. Clearly, the cleaner the delivery, the faster and more economical the microbonding operation can be. Adhesives used in microelectronics applications are typically loaded at levels up to 90% by weight with electrically or thermally conductive fillers, such as, silver, gold, copper, nickel, or silica. If any residual acid is left in the glycidyl ester, even at low levels, the acid has been found to cause a significant decrease in the thixotropic index of an adhesive containing a filler, particularly, for example, silver, in just 24 hours.
Thus, despite the fact that flexible epoxies are known and available, none meet all the requirements for a superior epoxy adhesive for use in microelectronics applications, namely, a high oxirane content, low ionic contamination, retention of a stable and workable viscosity, and when loaded with electrically or thermally conductive fillers a high, stable thixotropic index.